[css-env-1] Allowing embedding contexts to set env variables in embedded documents

[jwatt]

https://drafts.csswg.org/css-env-1/

The SVG Parameters doc has some ideas for allowing embedding elements to set CSS cascading variables in an embedded SVG, which is useful for doing things like theming. It seems to me that setting CSS environment variables would be more appropriate though. It also seems like this spec would be the place to specify the mechanism for doing this.

@dirkschulze @upsuper

[dirkschulze]

@jwatt SVG Parameters is not actively worked on. If CSS Env is super setting it then this is fine.